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    <title>Authentication, Authorization, and Access Control</title>
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<h1 align="center">Authentication, Authorization, and Access
Control</h1>

    <a name="TOC"></a>

    <ul>
      <li><a href="#intro">Introduction</a></li>
      <li>
        <a href="#basic">Basic authentication</a>

        <ul>
          <li><a href="#basicworks">How basic
          authentication works</a></li>

          <li>
            <a href="#basicconfig">Configuration:
            Protecting content with basic authentication</a>

          </li>

          <li>
            <a href="#basicfaq">Frequently asked
            questions about basic auth</a>
          </li>

          <li><a href="#basiccaveat">Security
          caveat</a></li>
        </ul>
        <br />
      </li>

      <li>
        <a href="#digest">Digest authentication</a>

        <ul>
          <li><a href="#digestworks">How digest auth
          works</a></li>

          <li>
            <a href="#digestconfig">Configuration:
            Protecting content with digest authentication</a>
          </li>

          <li><a href="#digestcaveat">Caveats</a></li>
        </ul>
        <br />
      </li>

      <li>
        <a href="#database">Database authentication
        modules</a>

        <ul>
          <li><a href="#modauthdb">mod_auth_db and
          mod_auth_dbm</a></li>

          <li><a href="#dbfiles">Berkeley DB files</a></li>

          <li><a href="#installauthdb">Installing mod_auth_db</a></li>

          <li>
            <a href="#authdbconfig">Protecting a
            directory with mod_auth_db</a>
          </li>
        </ul>
        <br />
      </li>

      <li>
        <a href="#access">Access control</a>

        <ul>
          <li><a href="#allowdeny">Allow and Deny</a></li>

          <li><a href="#satisfy">Satisfy</a></li>
        </ul>
        <br />
      </li>

      <li><a href="#summary">Summary</a></li>
    </ul>
    <!--End of Table of Child-Links-->
    <hr />

    <h1><a name="auth"></a><br />
     Authentication, Authorization, and Access Control</h1>

    <h1><a name="intro">Introduction</a></h1>

    <p>Apache has three distinct ways of dealing with the question
    of whether a particular request for a resource will result in
    that resource actually be returned. These criteria are called
    <i>Authorization</i>, <i>Authentication</i>, and <i>Access
    control</i>.</p>

    <p>Authentication is any process by which you verify that
    someone is who they claim they are. This usually involves a
    username and a password, but can include any other method of
    demonstrating identity, such as a smart card, retina scan,
    voice recognition, or fingerprints. Authentication is
    equivalent to showing your drivers license at the ticket
    counter at the airport.</p>

    <p>Authorization is finding out if the person, once identified,
    is permitted to have the resource. This is usually determined
    by finding out if that person is a part of a particular group,
    if that person has paid admission, or has a particular level of
    security clearance. Authorization is equivalent to checking the
    guest list at an exclusive party, or checking for your ticket
    when you go to the opera.</p>

    <p>Finally, access control is a much more general way of
    talking about controlling access to a web resource. Access can
    be granted or denied based on a wide variety of criteria, such
    as the network address of the client, the time of day, the
    phase of the moon, or the browser which the visitor is using.
    Access control is analogous to locking the gate at closing
    time, or only letting people onto the ride who are more than 48
    inches tall - it's controlling entrance by some arbitrary
    condition which may or may not have anything to do with the
    attributes of the particular visitor.</p>

    <p>Because these three techniques are so closely related in
    most real applications, it is difficult to talk about them
    separate from one another. In particular, authentication and
    authorization are, in most actual implementations,
    inextricable.</p>

    <p>If you have information on your web site that is sensitive,
    or intended for only a small group of people, the techniques in
    this tutorial will help you make sure that the people that see
    those pages are the people that you wanted to see them.</p>

    <h1><a name="basic"></a>Basic authentication</h1>

    <p>As the name implies, basic authentication is the simplest
    method of authentication, and for a long time was the most
    common authentication method used. However, other methods of
    authentication have recently passed basic in common usage, due
    to usability issues that will be discussed in a minute.</p>

    <h2><a name="basicworks"></a><br />
     How basic authentication works</h2>

    <p>When a particular resource has been protected using basic
    authentication, Apache sends a <tt>401 Authentication
    Required</tt> header with the response to the request, in order
    to notify the client that user credentials must be supplied in
    order for the resource to be returned as requested.</p>

    <p>Upon receiving a <tt>401</tt> response header, the client's
    browser, if it supports basic authentication, will ask the user
    to supply a username and password to be sent to the server. If
    you are using a graphical browser, such as Netscape or Internet
    Explorer, what you will see is a box which pops up and gives
    you a place to type in your username and password, to be sent
    back to the server. If the username is in the approved list,
    and if the password supplied is correct, the resource will be
    returned to the client.</p>

    <p>Because the HTTP protocol is stateless, each request will be
    treated in the same way, even though they are from the same
    client. That is, every resource which is requested from the
    server will have to supply authentication credentials over
    again in order to receive the resource.</p>

    <p>Fortunately, the browser takes care of the details here, so
    that you only have to type in your username and password one
    time per browser session - that is, you might have to type it
    in again the next time you open up your browser and visit the
    same web site.</p>

    <p>Along with the <tt>401</tt> response, certain other
    information will be passed back to the client. In particular,
    it sends a name which is associated with the protected area of
    the web site. This is called the <i>realm</i>, or just the
    authentication name. The client
    browser caches the username and password that you supplied, and
    stores it along with the authentication realm, so that if other
    resources are requested from the same realm, the same username
    and password can be returned to authenticate that request
    without requiring the user to type them in again. This caching
    is usually just for the current browser session, but some
    browsers allow you to store them permanently, so that you never
    have to type in your password again.</p>

    <p>The authentication name, or realm, will appear in the pop-up
    box, in order to identify what the username and password are
    being requested for.</p>

    <h2><a name="basicconfig"></a>
     Configuration: Protecting content with basic
    authentication</h2>

    <p>There are two configuration steps which you must complete in
    order to protect a resource using basic authentication. Or
    three, depending on what you are trying to do.</p>

    <ol>
      <li>Create a password file</li>

      <li>Set the configuration to use this password file</li>

      <li>Optionally, create a group file</li>
    </ol>

    <h3><a name="htpasswd"></a><br />
     Create a password file</h3>

    <p>In order to determine whether a particular username/password
    combination is valid, the username and password supplied by the
    user will need to be compared to some authoritative listing of
    usernames and password. This is the password file, which you
    will need to create on the server side, and populate with valid
    users and their passwords.</p>

    <p>Because this file contains sensitive information, it should
    be stored outside of the document directory. Although, as you
    will see in a moment, the passwords are encrypted in the file,
    if a cracker were to gain access to the file, it would be an
    aid in their attempt to figure out the passwords. And, because
    people tend to be sloppy with the passwords that they choose,
    and use the same password for web site authentication as for
    their bank account, this potentially be a very serious breach
    of security, even if the content on your web site is not
    particularly sensitive.</p>

    <p><b>Caution:</b> Encourage your users to use a different
    password for your web site than for other more essential
    things. For example, many people tend to use two passwords -
    one for all of their extremely important things, such as the
    login to their desktop computer, and for their bank account,
    and another for less sensitive things, the compromise of which
    would be less serious.</p>

    <p>To create the password file, use the <tt>htpasswd</tt>
    utility that came with Apache. This will be located in the
    <tt>bin</tt> directory of wherever you installed Apache. For
    example, it will probably be located at
    <tt>/usr/local/apache/bin/htpasswd</tt> if you installed Apache
    from source.</p>

    <p>To create the file, type:</p>
<pre>
htpasswd -c /usr/local/apache/passwd/passwords username
</pre>

    <p><tt>htpasswd</tt> will ask you for the password, and then
    ask you to type it again to confirm it:</p>
<pre>
# htpasswd -c /usr/local/apache/passwd/passwords rbowen
New password: mypassword
Re-type new password: mypassword
Adding password for user rbowen
</pre>

    <p>Note that in the example shown, a password file is being
    created containing a user called <tt>rbowen</tt>, and this
    password file is being placed in the location
    <tt>/usr/local/apache/passwd/passwords</tt>. You will
    substitute the location, and the username, which you want to
    use to start your password file.</p>

    <p>If <tt>htpasswd</tt> is not in your path, you will have to
    type the full path to the file to get it to run. That is, in
    the example above, you would replace <tt>htpasswd</tt> with
    <tt>/usr/local/apache/bin/htpasswd</tt></p>

    <p>The <tt>-c</tt> flag is used only when you are creating a
    new file. After the first time, you will omit the <tt>-c</tt>
    flag, when you are adding new users to an already-existing
    password file.</p>
<pre>
htpasswd /usr/local/apache/passwd/passwords sungo
</pre>

    <p>The example just shown will add a user named <tt>sungo</tt>
    to a password file which has already been created earlier. As
    before, you will be asked for the password at the command line,
    and then will be asked to confirm the password by typing it
    again.</p>

    <p><b>Caution:</b> Be very careful when you add new users to an
    existing password file that you don't use the <tt>-c</tt> flag
    by mistake. Using the <tt>-c</tt> flag will create a new
    password file, even if you already have an existing file of
    that name. That is, it will remove the contents of the file
    that is there, and replace it with a new file containing only
    the one username which you were adding.</p>

    <p>The password is stored in the password file in encrypted
    form, so that users on the system will not be able to read the
    file and immediately determine the passwords of all the users.
    Nevertheless, you should store the file in as secure a location
    as possible, with whatever minimum permissions on the file so
    that the web server itself can read the file. For example, if
    your server is configured to run as user <tt>nobody</tt> and
    group <tt>nogroup</tt>, then you should set permissions on the
    file so that only the webserver can read the file and only
    root can write to it:</p>
<pre>
chown root.nogroup /usr/local/apache/passwd/passwords
chmod 640 /usr/local/apache/passwd/passwords
</pre>

    <p>On Windows, a similar precaution should be taken, changing
    the ownership of the password file to the web server user, so
    that other users cannot read the file.</p>

    <h3><a name="htpasswdconfig"></a><br />
     Set the configuration to use this password file</h3>

    <p>Once you have created the password file, you need to tell
    Apache about it, and tell Apache to use this file in order to
    require user credentials for admission. This configuration is
    done with the following directives:</p>

    <table cellpadding="3">
      <tr>
        <td align="left">AuthType</td>

        <td align="left" valign="top" width="360">Authentication
        type being used. In this case, it will be set to
        <tt>Basic</tt></td>
      </tr>

      <tr>
        <td align="left">AuthName</td>

        <td align="left" valign="top" width="360">The
        authentication realm or name</td>
      </tr>

      <tr>
        <td align="left">AuthUserFile</td>

        <td align="left" valign="top" width="360">The location of
        the password file</td>
      </tr>

      <tr>
        <td align="left">AuthGroupFile</td>

        <td align="left" valign="top" width="360">The location of
        the group file, if any</td>
      </tr>

      <tr>
        <td align="left">Require</td>

        <td align="left" valign="top" width="360">The
        requirement(s) which must be satisfied in order to grant
        admission</td>
      </tr>
    </table>

    <p>These directives may be placed in a <tt>.htaccess</tt> file
    in the particular directory being protected, or may go in the
    main server configuration file, in a <tt>&lt;Directory&gt;</tt>
    section, or other scope container.</p>

    <p>The example shown below defines an authentication realm
    called ``By Invitation Only''. The password file located at
    <tt>/usr/local/apache/passwd/passwords</tt> will be used to
    verify the user's identity. Only users named <tt>rbowen</tt> or
    <tt>sungo</tt> will be granted access, and even then only if
    they provide a password which matches the password stored in
    the password file.</p>
<pre>
AuthType Basic
AuthName "By Invitation Only"
AuthUserFile /usr/local/apache/passwd/passwords
Require user rbowen sungo
</pre>

    <p>The phrase ``By Invitation Only'' will be displayed in the
    password pop-up box, where the user will have to type their
    credentials.</p>

    <p>You will need to restart your Apache server in order for the
    new configuration to take effect, if these directives were put
    in the main server configuration file. Directives placed in
    <tt>.htaccess</tt> files take effect immediately, since
    <tt>.htaccess</tt> files are parsed each time files are
    served.</p>

    <p>The next time that you load a file from that directory, you
    will see the familiar username/password dialog box pop up,
    requiring that you type the username and password before you
    are permitted to proceed.</p>

    <p>Note that in addition to specifically listing the users to
    whom you want to grant access, you can specify that any valid
    user should be let in. This is done with the
    <tt>valid-user</tt> keyword:</p>
<pre>
Require valid-user
</pre>

    <h3><a name="basicgroupfile"></a><br />
     Optionally, create a group file</h3>

    <p>Most of the time, you will want more than one, or two, or
    even a dozen, people to have access to a resource. You want to
    be able to define a group of people that have access to that
    resource, and be able to manage that group of people, adding
    and removing members, without having to edit the server
    configuration file, and restart Apache, each time.</p>

    <p>This is handled using authentication groups. An
    authentication group is, as you would expect, a group name
    associated with a list of members. This list is stored in a
    group file, which should be stored in the same location as the
    password file, so that you are able to keep track of these
    things.</p>

    <p>The format of the group file is exceedingly simple. A group
    name appears first on a line, followed by a colon, and then a
    list of the members of the group, separated by spaces. For
    example:</p>
<pre>
authors: rich daniel allan
</pre>

    <p>Once this file has been created, you can <tt>Require</tt>
    that someone be in a particular group in order to get the
    requested resource. This is done with the
    <tt>AuthGroupFile</tt> directive, as shown in the following
    example.</p>
<pre>
AuthType Basic
AuthName "Apache Admin Guide Authors"
AuthUserFile /usr/local/apache/passwd/passwords
AuthGroupFile /usr/local/apache/passwd/groups
Require group authors
</pre>

    <p>The authentication process is now one step more involved.
    When a request is received, and the requested username and
    password are supplied, the group file is first checked to see
    if the supplied username is even in the required group. If it
    is, then the password file will be checked to see if the
    username is in there, and if the supplied password matches the
    password stored in that file. If any of these steps fail,
    access will be forbidden.</p>

    <h2><a name="basicfaq"></a><br />
     Frequently asked questions about basic auth</h2>

    <p>The following questions tend to get asked very frequently
    with regard to basic authentication. It should be understood
    that basic authentication is very basic, and so is limited to
    the set of features that has been presented above. Most of the
    more interesting things that people tend to want, need to be
    implemented using some alternate authentication scheme.</p>

    <h3><a name="logout"></a><br />
     How do I log out?</h3>

    <p>Since browsers first started implementing basic
    authentication, website administrators have wanted to know how
    to let the user log out. Since the browser caches the username
    and password with the authentication realm, as described
    earlier in this tutorial, this is not a function of the server
    configuration, but is a question of getting the browser to
    forget the credential information, so that the next time the
    resource is requested, the username and password must be
    supplied again. There are numerous situations in which this is
    desirable, such as when using a browser in a public location,
    and not wishing to leave the browser logged in, so that the
    next person can get into your bank account.</p>

    <p>However, although this is perhaps the most frequently asked
    question about basic authentication, thus far none of the major
    browser manufacturers have seen this as being a desirable
    feature to put into their products.</p>

    <p>Consequently, the answer to this question is, you can't.
    Sorry.</p>

    <h3><a name="passworddialog"></a><br />
     How can I change what the password box looks like?</h3>

    <p>The dialog that pops up for the user to enter their username
    and password is ugly. It contains text that you did not
    indicate that you wanted in there. It looks different in
    Internet Explorer and Netscape, and contains different text.
    And it asks for fields that the user might not understand -
    for example, Netscape asks the user to type in their ``User
    ID'', and they might not know what that means. Or, you might
    want to provide additional explanatory text so that the user
    has a better idea what is going on.</p>

    <p>Unfortunately, these things are features of the browser, and
    cannot be controlled from the server side. If you want the
    login to look different, then you will need to implement your
    own authentication scheme. There is no way to change what this
    login box looks like if you are using basic authentication.</p>

    <h3><a name="persistpass"></a><br />
     How to I make it not ask me for my password the next
    time?</h3>

    <p>Because most browsers store your password information only
    for the current browser session, when you close your browser it
    forgets your username and password. So, when you visit the same
    web site again, you will need to re-enter your username and
    password.</p>

    <p>There is nothing that can be done about this on the server
    side.</p>

    <p>However, the most recent versions of the major browsers
    contain the ability to remember your password forever, so that
    you never have to log in again. While it is debatable whether
    this is a good idea, since it effectively overrides the entire
    point of having security in the first place, it is certainly
    convenient for the user, and simplifies the user
    experience.</p>

    <h3><a name="passwordtwice"></a><br />
     Why does it sometimes ask me for my password twice?</h3>

    <p>When entering a password-protected web site for the first
    time, you will occasionally notice that you are asked for your
    password twice. This may happen immediately after you entered
    the password the first time, or it may happen when you click on
    the first link after authenticating the first time.</p>

    <p>This happens for a very simple, but nonetheless confusing,
    reason, again having to do with the way that the browser caches
    the login information.</p>

    <p>Login information is stored on the browser based on the
    authentication realm, specified by the <tt>AuthName</tt>
    directive, and by the server name. In this way, the browser can
    distinguish between the <tt>Private</tt> authentication realm
    on one site and on another. So, if you go to a site using one
    name for the server, and internal links on the server refer to
    that server by a different name, the browser has no way to know
    that they are in fact the same server.</p>

    <p>For example, if you were to visit the URL
    <tt>http://example.com/private/</tt>, which required
    authentication, your browser would remember the supplied
    username and password, associated with the hostname
    <tt>example.com</tt>. If, by virtue of an internal redirect, or
    fully-qualified HTML links in pages, you are then sent to the
    URL <tt>http://www.example.com/private/</tt>, even though this
    is really exactly the same URL, the browser does not know this
    for sure, and is forced to request the authentication
    information again, since <tt>example.com</tt> and
    <tt>www.example.com</tt> are not exactly the same hostname.
    Your browser has no particular way to know that these are the
    same web site.</p>

    <h2><a name="basiccaveat"></a><br />
     Security caveat</h2>

    <p>Basic authentication should not be considered secure for any
    particularly rigorous definition of secure.</p>

    <p>Although the password is stored on the server in encrypted
    format, it is passed from the client to the server in plain
    text across the network. Anyone listening with any variety of
    packet sniffer will be able to read the username and password
    in the clear as it goes across.</p>

    <p>Not only that, but remember that the username and password
    are passed with every request, not just when the user first
    types them in. So the packet sniffer need not be listening at a
    particularly strategic time, but just for long enough to see
    any single request come across the wire.</p>

    <p>And, in addition to that, the content itself is also going
    across the network in the clear, and so if the web site
    contains sensitive information, the same packet sniffer would
    have access to that information as it went past, even if the
    username and password were not used to gain direct access to
    the web site.</p>

    <p>Don't use basic authentication for anything that requires
    real security. It is a detriment for most users, since very few
    people will take the trouble, or have the necessary software
    and/or equipment, to find out passwords. However, if someone
    had a desire to get in, it would take very little for them to
    do so.</p>

    <p>Basic authentication across an SSL connection, however, will be
    secure, since everything is going to be encrypted, including the
    username and password.</p>

    <h1><a name="digest"></a>Digest authentication</h1>

    <p>Addressing one of the security caveats of basic
    authentication, digest authentication provides an alternate
    method for protecting your web content. However, it to has a
    few caveats.</p>

    <h2><a name="digestworks">How digest auth works</a></h2>

    <p>Digest authentication is implemented by the module
    <tt>mod_auth_digest</tt>. There is an older module,
    <tt>mod_digest</tt>, which implemented an older version of the
    digest authentication specification, but which will probably
    not work with newer browsers.</p>

    <p>Using digest authentication, your password is never sent
    across the network in the clear, but is always transmitted as
    an MD5 digest of the user's password. In this way, the password
    cannot be determined by sniffing network traffic.</p>

    <p>The full specification of digest authentication can be seen
    in the internet standards document RFC 2617, which you can see
    at <tt>http://www1.ics.uci.edu/pub/ietf/http/rfc2617.txt</tt>.
    Additional information and resources about MD5 can be found at
    <tt>http://userpages.umbc.edu/&nbsp;mabzug1/cs/md5/md5.html</tt></p>

    <h2><a name="digestconfig"></a>Configuration:
    Protecting content with digest authentication</h2>

    <p>The steps for configuring your server for digest
    authentication are very similar for those for basic
    authentication.</p>

    <ol>
      <li>Create the password file</li>

      <li>Set the configuration to use this password file</li>

      <li>Optionally, create a group file</li>
    </ol>

    <h3><a name="htdigest"></a>Creating a password file</h3>

    <p>As with basic authentication, a simple utility is provided
    to create and maintain the password file which will be used to
    determine whether a particular user's name and password are
    valid. This utility is called <tt>htdigest</tt>, and will be
    located in the <tt>bin</tt> directory of wherever you installed
    Apache. If you installed Apache from some variety of package
    manager, <tt>htdigest</tt> is likely to have been placed
    somewhere in your path.</p>

    <p>To create a new digest password file, type:</p>
<pre>
htdigest -c /usr/local/apache/passwd/digest realm username
</pre>

    <p><tt>htdigest</tt> will ask you for the desired password, and
    then ask you to type it again to confirm it.</p>

    <p>Note that the realm for which the authentication will be
    required is part of the argument list.</p>

    <p>Once again, as with basic authentication, you are encouraged
    to place the generated file somewhere outside of the document
    directory.</p>

    <p>And, as with the <tt>htpasswd</tt> utility, the <tt>-c</tt>
    flag creates a new file, or, if a file of that name already
    exists, deletes the contents of that file and generates a new
    file in its place. Omit the <tt>-c</tt> flag in order to add
    new user information to an existing password file.</p>

    <h3><a name="htdigestconfig"></a>Set the configuration
    to use this password file</h3>

    <p>Once you have created a password file, you need to tell
    Apache about it in order to start using it as a source of
    authenticated user information. This configuration is done with
    the following directives:</p>

    <table cellpadding="3">
      <tr>
        <td align="left">AuthType</td>

        <td align="left" valign="top" width="360">Authentication
        type being used. In this case, it will be set to
        <tt>Digest</tt></td>
      </tr>

      <tr>
        <td align="left">AuthName</td>

        <td align="left" valign="top" width="360">The
        authentication realm or name</td>
      </tr>

      <tr>
        <td align="left">AuthDigestFile</td>

        <td align="left" valign="top" width="360">The location of
        the password file</td>
      </tr>

      <tr>
        <td align="left">AuthDigestGroupFile</td>

        <td align="left" valign="top" width="360">Location of the
        group file, if any</td>
      </tr>

      <tr>
        <td align="left">Require</td>

        <td align="left" valign="top" width="360">The
        requirement(s) which must be satisfied in order to grant
        admission</td>
      </tr>
    </table>

    <p>These directives may be placed in a <tt>.htaccess</tt> file
    in the particular directory being protected, or may go in the
    main server configuration file, in a <tt>&lt;Directory&gt;</tt>
    section, or another scope container.</p>

    <p>The following example defines an authentication realm called
    "Private". The password file located at
    <tt>/usr/local/apache/passwd/digest</tt> will be used to verify
    the user's identity. Only users named <tt>drbacchus</tt> or
    <tt>dorfl</tt> will be granted access, if they provide a
    password that patches the password stored in the password
    file.</p>
<pre>
AuthType Digest
AuthName "Private"
AuthDigestFile /usr/local/apache/passwd/digest
Require user drbacchus dorfl
</pre>

    <p>The phrase "Private" will be displayed in the password
    pop-up box, where the user will have to type their
    credentials.</p>

    <h3><a name="digestgroup"></a>Optionally, create a group file</h3>

    <p>As you have observed, there are not many differences between
    this configuration process and that required by basic
    authentication, described in the previous section. This is true
    also of group functionality. The group file used for digest
    authentication is exactly the same as that used for basic
    authentication. That is to say, lines in the group file consist
    the name of the group, a colon, and a list of the members of
    that group. For example:</p>
<pre>
admins: jim roy ed anne
</pre>

    <p>Once this file has been created, you can <tt>Require</tt>
    that someone be in a particular group in order to get the
    requested resource. This is done with the
    <tt>AuthDigestGroupFile</tt> directive, as shown in the
    following example.</p>
<pre>
AuthType Digest
AuthName "Private"
AuthDigestFile /usr/local/apache/passwd/digest
AuthDigestGroupFile /usr/local/apache/passwd/digest.groups
Require group admins
</pre>

    <p>The authentication process is the same as that used by basic
    authentication. It is first verified that the user is in the
    required group, and, if this is true, then the password is
    verified.</p>

    <h2><a name="digestcaveat">Caveats</a></h2>

    <p>Before you leap into using digest authentication instead of
    basic authentication, there are a few things that you should
    know about.</p>

    <p>Most importantly, you need to know that, although digest
    authentication has this great advantage that you don't send
    your password across the network in the clear, it is not
    supported by all major browsers in use today, and so you should
    not use it on a web site on which you cannot control the
    browsers that people will be using, such as on your intranet
    site. In particular, Opera 4.0 or later, Microsoft Internet
    Explorer 5.0 or later, Mozilla 1.0.1 and Netscape 7 or later
    as well as Amaya support digest authentication, while various
    other browsers do not.</p>

    <p>Next, with regard to security considerations, you should
    understand two things. Although your password is not passed in
    the clear, all of your data is, and so this is a rather small
    measure of security. And, although your password is not really
    sent at all, but a digest form of it, someone very familiar
    with the workings of HTTP could use that information - just
    your digested password - and use that to gain access to the
    content, since that digested password is really all the
    information required to access the web site.</p>

    <p>The moral of this is that if you have content that really
    needs to be kept secure, use SSL.</p>

    <h1><a name="database">Database authentication
    modules</a></h1>

    <p>Basic authentication and digest authentication both suffer
    from the same major flaw. They use text files to store the
    authentication information. The problem with this is that
    looking something up in a text file is very slow. It's rather
    like trying to find something in a book that has no index. You
    have to start at the beginning, and work through it one page at
    a time until you find what you are looking for. Now imagine
    that the next time you need to find the same thing, you don't
    remember where it was before, so you have to start at the
    beginning again, and work through one page at a time until you
    find it again. And the next time. And the time after that.</p>

    <p>Since HTTP is stateless, authentication has to be verified
    every time that content is requested. And so every time a
    document is accessed which is secured with basic or digest
    authentication, Apache has to open up those text password files
    and look through them one line at a time, until it finds the
    user that is trying to log in, and verifies their password. In
    the worst case, if the username supplied is not in there at
    all, every line in the file will need to be checked. On
    average, half of the file will need to be read before the user
    is found. This is very slow.</p>

    <p>While this is not a big problem for small sets of users,
    when you get into larger numbers of users (where "larger" means
    a few hundred) this becomes prohibitively slow. In many cases,
    in fact, valid username/password combinations will get rejected
    because the authentication module just had to spend so much
    time looking for the username in the file that Apache will just
    get tired of waiting and return a failed authentication.</p>

    <p>In these cases, you need an alternative, and that
    alternative is to use some variety of database. Databases are
    optimized for looking for a particular piece of information in
    a very large data set. It builds indexes in order to rapidly
    locate a particular record, and they have query languages for
    swiftly locating records that match particular criteria.</p>

    <p>There are numerous modules available for Apache to
    authenticate using a variety of different databases. In this
    section, we'll just look at two modules which ship with Apache.
    </p>

    <h2><a name="modauthdb"></a>mod_auth_db and mod_auth_dbm</h2>

    <p><tt>mod_auth_db</tt> and <tt>mod_auth_dbm</tt> are modules
    which lets you keep your usernames and passwords in DB or DBM
    files. There are few practical differences between DB files and
    DBM files. And, on some operating systems, such as various
    BSDs, and Linux, they are exactly the same thing. You should
    pick whichever of the two modules makes the most sense on your
    particular platform of choice. If you do not have DB support on
    your platform, you may need to install it. You download an
    implementation of DB at <tt>http://www.sleepycat.com/</tt>.

    <h2><a name="dbfiles"></a>Berkeley DB files</h2>

    <p>DB files, also known as Berkeley database files, are the
    simplest form of database, and are rather ideally suited for
    the sort of data that needs to be stored for HTTP
    authentication. DB files store key/value pairs. That is, the
    name of a variable, and the value of that variable. While other
    databases allow the storage of many fields in a given record, a
    DB file allows only this pairing of key and value.<a
    name="foot1_return" href="#foot1"><sup>1</sup></a> This is ideal for
    authentication, which requires only the pair of a username and
    password.</p>

    <h2><a name="installauthdb">Installing mod_auth_db</a></h2>

    <p>For the purposes of this tutorial, we'll talk about
    installing and configuring <tt>mod_auth_db</tt>. However,
    everything that is said here can be directly applied to
    <tt>mod_auth_dbm</tt> by simply replacing 'db' with 'dbm' and
    'DB' with 'DBM' in the various commands, file names, and
    directives.</p>

    <p>Since <tt>mod_auth_db</tt> is not compiled in by default,
    you will need to rebuild Apache in order to get the
    functionality, unless you built in everything when we started.
    Note that if you installed Apache with shared object
    support, you may be able to just build the module and load it
    in to Apache.</p>

    <p>To build Apache from scratch with <tt>mod_auth_db</tt> built
    in, use the following <tt>./configure</tt> line in your apache
    source code directory.</p>
<pre>
./configure --enable-module=auth_db
</pre>

    <p>Or, if you had a more complex <tt>configure</tt> command
    line, you can just add the <tt>-enable-module=auth_db</tt>
    option to that command line, and you'll get
    <tt>mod_auth_db</tt> built into your server.</p>

    <h2><a name="authdbconfig">Protecting a directory with
    mod_auth_db</a></h2>

    <p>Once you have compiled the <tt>mod_auth_db</tt> module, and
    loaded it into your web server, you'll find that there's very
    little difference between using regular authentication and
    using <tt>mod_auth_db</tt> authentication. The procedure is the
    same as that we went through with basic and digest
    authentication:</p>

    <ol>
      <li>Create the user file.</li>

      <li>Configure Apache to use that file for
      authentication.</li>

      <li>Optionally, create a group file.</li>
    </ol>

    <h3><a name="dbmmanage"></a>Create the user file</h3>

    <p>The user file for authentication is, this time, not a flat
    text file, but is a DB file<a name="foot2_return"
    href="#foot2"><sup>2</sup></a>. Fortunately, once again,
    Apache provides us with a simple utility for the purpose of
    managing this user file. This time, the utility is called
    <tt>dbmmanage</tt>, and will be located in the <tt>bin</tt>
    subdirectory of wherever you installed Apache.</p>

    <p><tt>dbmmanage</tt> is somewhat more complicated to use than
    <tt>htpasswd</tt> or <tt>htdigest</tt>, but it is still fairly
    simple. The syntax which you will usually be using is as
    follows:</p>
<pre>
dbmmanage passwords.dat adduser montressor
</pre>

    <p>As with <tt>htpasswd</tt>, you will at this point be
    prompted for a password, and then asked to confirm that
    password by typing it again. The main difference here is that
    rather than a text file being created, you are creating a
    binary file containing the information that you have
    supplied.</p>

    <p>Type <tt>dbmmanage</tt> with no arguments to get the full
    list of options available with this utility.</p>

    <h3><a name="perl_dbfile">Creating your user file with
    Perl</a></h3>

    <p>Note that, if you are so inclined, you can manage your user
    file with Perl, or any other language which has a DB-file
    module, for interfacing with this type of database. This covers
    a number of popular programming languages.</p>

    <p>The following Perl code, for example, will add a user
    'rbowen', with password 'mypassword', to your password
    file:</p>
<pre>
use DB_File;
tie %database, 'DB_File', "passwords.dat"
    or die "Can't initialize database: $!\n";

$username = 'rbowen';
$password = 'mypassword';
@chars=(0..9,'a'..'z');
$salt = $chars[int rand @chars] . $chars[int rand @chars];

$crypt = crypt($password, $salt);
$database{$username} = $crypt;

untie %database;
</pre>

    <p>As you can imagine, this makes it very simple to write tools
    to manage the user and password information stored in these
    files.</p>

    <p>Passwords are stored in Unix <tt>crypt</tt> format, just as
    they were in the "regular" password files. The 'salt' that is
    created in the middle there is part of the process, generating
    a random starting point for that encryption. The technique
    being used is called a 'tied hash'. The idea is to tie a
    built-in data structure to the contents of the file, such that
    when the data structure is changed, the file is automatically
    modified at the same time.</p>

    <h3><a name="authdbuserfile"></a>Configuring Apache
    to use this password file</h3>

    <p>Once you have created the password file, you need to tell
    Apache about it, and tell Apache to use this file to verify
    user credentials. This configuration will look almost the same
    as that for basic authentication. This configuration can go in
    a <tt>.htaccess</tt> file in the directory to be protected, or
    can go in the main server configuration, in a
    <tt>&lt;Directory&gt;</tt> section, or other scope container
    directive.</p>

    <p>The configuration will look something like the
    following:</p>
<pre>
AuthName "Members Only"
AuthType Basic
AuthDBUserFile /usr/local/apache/passwd/passwords.dat
require user rbowen
</pre>

    <p>Now, users accessing the directory will be required to
    authenticate against the list of valid users who are in
    <tt>/usr/local/apache/passwd/passwords.dat</tt>.</p>

    <h3><a name="authdbgroupfile"></a><br />
     Optionally, create a group file</h3>

    <p>As mentioned earlier, DB files store a key/value pair. In
    the case of group files, the key is the name of the user, and
    the value is a comma-separated list of the groups to which the
    user belongs.</p>

    <p>While this is the opposite of the way that group files are
    stored elsewhere, note that we will primarily be looking up
    records based on the username, so it is more efficient to index
    the file by username, rather than by the group name.</p>

    <p>Groups can be added to your group file using
    <tt>dbmmanage</tt> and the <tt>add</tt> command:</p>
<pre>
dbmmanage add groupfile rbowen one,two,three
</pre>

    <p>In the above example, <tt>groupfile</tt> is the literal name
    of the group file, <tt>rbowen</tt> is the user being added, and
    <tt>one</tt>, <tt>two</tt>, and <tt>three</tt> are names of
    three groups to which this user belongs.</p>

    <p>Once you have your groups in the file, you can require a
    group in the regular way:</p>
<pre>
AuthName "Members Only"
AuthType Basic
AuthDBUserFile /usr/local/apache/passwd/passwords.dat
AuthDBGroupFile /usr/local/apache/passwd/groups.dat
require group three
</pre>

    <p>Note that if you want to use the same file for both password
    and group information, you can do so, but this is a little more
    complicated to manage, as you have to encrypt the password
    yourself before you feed it to the <tt>dbmmanage</tt>
    utility.</p>

    <h1><a name="access"></a>Access control</h1>

    <p>Authentication by username and password is only part of the
    story. Frequently you want to let people in based on something
    other than who they are. Something such as where they are
    coming from. Restricting access based on something other than
    the identity of the user is generally referred to as <i>Access
    Control</i>.</p>

    <h2><a name="allowdeny"></a>Allow and Deny</h2>

    <p>The <tt>Allow</tt> and <tt>Deny</tt> directives let you
    allow and deny access based on the host name, or host address,
    of the machine requesting a document. The directive goes
    hand-in-hand with these is the <tt>Order</tt> directive, which
    tells Apache in which order to apply the filters.</p>

    <p>The usage of these directives is:</p>
<pre>
allow from address
</pre>

    <p>where <i>address</i> is an IP address (or a partial IP
    address) or a fully qualified domain name (or a partial domain
    name); you may provide multiple addresses or domain names, if
    desired.</p>

    <p>For example, if you have someone spamming your message
    board, and you want to keep them out, you could do the
    following:</p>
<pre>
deny from 11.22.33.44
</pre>

    <p>Visitors coming from that address will not be able to see
    the content behind this directive. If, instead, you have a
    machine name, rather than an IP address, you can use that.
    </p>
<pre>
deny from hostname.example.com
</pre>

    <p>And, if you'd like to block access from an entire domain,
    or even from an entire tld (top level domain, such as .com or .gov)
    you can specify just part of an address or domain name:</p>
<pre>
deny from 192.101.205
deny from exampleone.com exampletwo.com
deny from tld
</pre>

    <p>Using <tt>Order</tt> will let you be sure that you are
    actually restricting things to the group that you want to let
    in, by combining a <tt>deny</tt> and an <tt>allow</tt>
    directive:</p>
<pre>
Order Deny,Allow
Deny from all
Allow from hostname.example.com
</pre>

    <p>Listing just the <tt>allow</tt> directive would not do what
    you want, because it will let users from that host in, in
    addition to letting everyone in. What you want is to let in
    <i>only</i> users from that host.</p>

    <h2><a name="satisfy"></a>Satisfy</h2>

    <p>The <tt>Satisfy</tt> directive can be used to specify that
    several criteria may be considered when trying to decide if a
    particular user will be granted admission. <tt>Satisfy</tt> can
    take as an argument one of two options - <tt>all</tt> or
    <tt>any</tt>. By default, it is assumed that the value is
    <tt>all</tt>. This means that if several criteria are
    specified, then all of them must be met in order for someone to
    get in. However, if set to <tt>any</tt>, then several criteria
    may be specified, but if the user satisfies any of these, then
    they will be granted entrance.</p>

    <p>A very good example of this is using access control to
    assure that, although a resource is password protected from
    outside your network, all hosts inside the network will be
    given free access to the resource. This would be accomplished
    by using the <tt>Satisfy</tt> directive, as shown below.</p>
<pre>
&lt;Directory /usr/local/apache/htdocs/sekrit&gt;
  AuthType Basic
  AuthName intranet
  AuthUserFile /www/passwd/users
  AuthGroupFile /www/passwd/groups
  Require group customers
  Order allow,deny
  Allow from internal.com
  Satisfy any
&lt;/Directory&gt;
</pre>

    <p>In this scenario, users will be let in if they either have a
    password, or if they are in the internal network.</p>

    <h1><a name="summary">Summary</a></h1>

    <p>The various authentication modules provide a number of ways
    to restrict access to your host based on the identity of the
    user. They offer a somewhat standard interface to this
    functionality, but provide different back-end mechanisms for
    actually authenticating the user.</p>

    <p>And the access control mechanism allows you to restrict
    access based on criteria unrelated to the identity of the
    user.<br />
    </p>
    <hr />

    <h4>Footnotes</h4>

    <dl>
      <dt><a name="foot1">... value.</a><a
      href="#foot1_return"><sup>1</sup></a></dt>

      <dd>There are actually a number of implementations that get
      around this limitation. MLDBM is one of them, for example.
      However, for the purposes of this discussion, we'll just deal
      with standard Berkeley DB, which is likely to have shipped
      with whatever operating system you are already running.</dd>

      <dt><a name="foot2">... file</a><a
      href="#foot2_return"><sup>2</sup></a></dt>

      <dd>Or, if you are using mod_auth_dbm, a DBM file.</dd>
    </dl>
    <hr />

  </body>
</html>
